Presentation for effect systems meetup on evidence passing

effects-july1-2023/Makefile

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+all:
+	make slide slideshow
+
+slide:
+	pandoc -t beamer --include-in-header=./style.tex effects-evidence.md -f markdown-implicit_figures -V colorlinks=true -V linkcolor=blue -V urlcolor=red -o effects-evidence.pdf
+
+slideshow:
+	pandoc -t beamer --include-in-header=./style.tex effects-evidence.md -f markdown-implicit_figures -V colorlinks=true -V linkcolor=blue -V urlcolor=red -i -o effects-evidence-slideshow.pdf
+
+view:
+	zathura --mode=presentation effects-evidence.pdf &

effects-july1-2023/effects-evidence.md

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+---
+title:
+- Effect Systems in Haskell - Part II
+author:
+- Sanchayan Maity
+theme:
+- default
+classoption:
+- aspectratio=169
+---
+
+# Agenda
+
+- Cover two papers on Effect Systems.
+    * Generalized Evidence Passing for Effect Handlers[^1]
+    * Effect Handlers in Haskell, Evidently[^2]
+- Some sections today's discussion isn't going to cover
+    * Efficiency/Performance of the library or effect system itself
+    * How to use effect systems
+    * Comparison of effect system libraries or how to choose one
+
+
+[^1]: [Generalized Evidence Passing for Effect Handlers](https://dl.acm.org/doi/pdf/10.1145/3473576)
+[^2]: [Effect Handlers in Haskell, evidently](https://xnning.github.io/papers/haskell-evidently.pdf)
+
+# Recap, what's it all about
+
+- **Separate syntax from semantics**
+- **Interpret your abstract syntax tree in various ways**
+- **Not losing performance while having both**
+
+# Recap, why effect systems
+
+- Monads to model effects but monads don't compose[^3]
+- transformers/mtl has limitations
+    * Monad transformer stacks are rigid
+    * Doesn't allow handling something like `Reader Int (Reader String)` due to functional dependencies
+    ```haskell
+    class Monad m => MonadReader r m | m -> r
+    ```
+
+    * More than a few effects in stack become unwieldy
+    * n-square instances problem
+
+[^3]: [Composing Monads by Mark Jones and Luc Duponcheel](https://web.cecs.pdx.edu/~mpj/pubs/RR-1004.pdf)
+
+# Evidence passing based effect system libraries
+
+- `EvEff`
+- `MpEff`
+- `speff`
+- `cleff` based on `ReaderT IO`
+- `effectful` based on `ReaderT IO`
+- others?
+
+# Some myths[^4]
+
+- Many extensible effects libraries are implemented with free(r) monads (True)
+- Therefore extensible effects = free(r) monads (False)
+- Free(r) monads require certain mathematical concepts to grasp (True)
+- Free(r) monads don't have very good performance (True, to some extent)
+- Therefore extensible effects are slow, ivory-towerish toys (False)
+
+[^4]: [ReaderT pattern is just extensible effects](https://喵.世界/2022/02/03/readert-is-extensible-effects/)
+
+# Effects can be implemented in various way
+
+- `free` monads
+- `ReaderT IO`
+- `CPS`
+- `delimited continuations`
+
+# What's the gist
+
+- How do you pass the `handler` for the `effect`?
+
+# Generalized control flow
+
+- Languages that expose a yield primitive actually have a way to access delimited continuations! Central result of the paper by James-Sabry [^5][^6].
+
+[^5]: [Yield: Mainstream Delimited Continuations](https://legacy.cs.indiana.edu/~sabry/papers/yield.pdf)
+[^6]: [Delimited Continuations are all you need](https://www.youtube.com/watch?v=uRbqLGj_6mI)
+
+# WTH are delimited continuations
+
+- Delimited Continuations for Everyone[^7]
+
+[^7]: [Delimited Continuations for Everyone](https://github.com/papers-we-love/pwlconf-info/tree/master/2017/kenichi-asai)
+
+# How does one define an effect
+
+```haskell
+data Reader a e ans = Reader { ask :: !(Op () a e ans)
+                             }
+
+data State a e ans = State { get :: !(Op () a e ans)
+                           , put :: !(Op a () e ans)
+                           }
+```
+
+# Multi-prompt delimited control
+
+```haskell
+data Ctl e a = Pure { result :: !a }
+             | forall h b e' ans.
+               Control {
+      -- prompt marker to yield to (in type context `::ans`)
+                  marker :: Marker h e' ans,
+      -- the final action, just needs the resumption (:: b -> Eff e' ans) to be evaluated.
+                op     :: !((b -> Eff e' ans) -> Eff e' ans),
+      -- the (partially) build up resumption;
+      -- (b -> Eff e a) :~: (b -> Eff e' ans)` by the time
+      -- we reach the prompt
+                  cont   :: !(b -> Eff e a) }
+
+data Context e where
+  CCons :: !(Marker h e' ans) -> !(h e' ans) -> !(ContextT e e')
+          -> !(Context e) -> Context (h :* e)
+  CNil  :: Context ()
+
+newtype Eff e a = Eff { unEff :: Context e -> Ctl e a }
+```
+
+# Key ideas
+
+- Multi Prompt
+- Evidence Passing
+- Tail Resumptive Operations
+- Bubbling Yields
+- Short cut resumptions
+- Monadic Translation
+- Bind-inlining and Join-Point Sharing
+
+# Dig in
+
+- Dig in to the paper!
+
+# Resources
+
+- [Alexis King on "Delimited Continuations, Demystified" @ZuriHac2023](https://www.youtube.com/watch?v=DRFsodbxHQo)
+- [GHC Proposal: Delimited continuation primops](https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0313-delimited-continuation-primops.rst)
+- [Delimited Continuations](https://calwoo.github.io/posts/2020-02-03-delimited.html)
+- [Efficient Compilation of Algebraic Effect Handlers - Ningning Xie](https://www.youtube.com/watch?v=tWLPrPfb4_U)
+- [From Folklore to Fact: Comparing Implementations of Stacks and Continuations](https://kavon.farvard.in/papers/pldi20-stacks.pdf)
+- [Compiler and Runtime Support for Continuation Marks](https://www-old.cs.utah.edu/plt/publications/pldi20-fd.pdf)
+- [Capturing the Future by Replaying the Past Functional Pearl](https://arxiv.org/pdf/1710.10385.pdf)
+- [From Delimited Continuations to Algebraic Effects in Haskell](https://blog.poisson.chat/posts/2023-01-02-del-cont-examples.html)
+- [Concurrent System Programming with Effect Handlers](https://kcsrk.info/papers/system_effects_feb_18.pdf)
+- [Eff Directly in OCaml](https://arxiv.org/pdf/1812.11664.pdf)
+- [Retrofitting Effect Handlers onto OCaml](https://kcsrk.info/papers/drafts/retro-concurrency.pdf)
+
+# Questions?
+
+- Reach out on
+  * Email: sanchayan@sanchayanmaity.net
+  * Mastodon: https://sanchayanmaity.com/@sanchayan
+  * Blog: https://sanchayanmaity.net
+  * Telegram:
+    - t.me/fpncr
+    - t.me/SanchayanMaity

effects-july1-2023/style.tex

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+\logo{\includegraphics[height=0.5cm]{../images/Haskell.jpg}\vspace{220pt}\hspace{8pt}}